Ans:
Plastic materials can be classified into thermoplastic and thermosetting plastics.
Thermoplastic materials:- The properties of these plastic materials do not change
considerably if they are melted and then cooled and solidify. They can be repeatedly melted
or dissolved in various solvents. They are more elastic, less brittle and do not lose elasticity
when subjected to prolonged heating. They are less apt to age thermally. They can be
remoulded again and again in any shape after heating. Many of them possess extraordinary
high insulating properties and are water repellent. They are polymers of linear structure, i.e.
their molecules are elongated and are thread like. This, type of structure is fusible, soluble,
highly plastic, capable of forming thin flexible threads and films. Examples are Polytetra
Flouroethylene (P.T.F.E. or Teflon), Polyvinyl Chloride (P.V.C.).
Thermosetting Plastic Materials:- They undergo great changes when subjected to high
temperatures for quite sometimes. They are said to be baked and no longer can melt or be
dissolved. They are less elastic, more brittle and lose their elasticity when subjected to
prolonged heating. So they cannot be remoulded in different shapes once they are set and
hardened. They are used, when an insulation is to withstand high temperatures without
melting or losing its shape and mechanical strength. Thermosetting plastic substances are
space-polymers and the molecules branch off in various directions during polymerisation.
This structure makes them very rigid, poorly soluble, fusible and incapable of forming elastic
threads and films. Examples are Phenol formaldehyde (Bakelite), Epoxy resins.